Recently, in applications such as label package doubling as a protection of a glass bottle and a PET bottle etc. and display of articles, cap sealing and accumulation package, there have been widely used drawn films (so-called heat-shrinkable films) composed of a polyvinyl chloride resin, a polystyrene resin, a polyethylene resin or the like. Of these heat-shrinkable films, a polyvinyl chloride film has problems that heat resistance is low, and it generates hydrogen chloride gas in incineration and causes dioxin. A polystyrene film has problems that it is inferior in chemical resistance, as well as an ink with a special composition needs to be used in printing, it requires high temperature incineration and generates a lot of black smoke accompanied by an abnormal odor. Therefore, as a shrink label, there has been widely used a polyester-based heat-shrinkable film which is high in heat resistance, easy to incinerate, and excellent in chemical resistance, and the use amount tends to increase being accompanied by an increase in turn volume of PET containers.
As a heat-shrinkable polyester film, one which is allowed to greatly shrink in the width direction has been widely utilized so far. Although the heat-shrinkable polyester film in which the width direction is the main shrinkage direction has been subjected to drawing at a high ratio in the width direction in order to allow the shrinkage properties in the width direction to be developed, with regard to the longitudinal direction orthogonal to the main shrinkage direction, there have been many cases in which the film is only subjected to drawing at a low ratio and there is also a case in which the film is not subjected to drawing. Such a film subjected to drawing at a low ratio in the longitudinal direction and a film subjected to drawing only in the width direction have a drawback that the mechanical strength in the longitudinal direction is poor.
Further, as a heat-shrinkable film, a film is generally utilized which greatly shrinks in the width direction in terms of handleability in label production. Hence, the conventional heat-shrinkable polyester film has been produced by drawing at a high ratio in the width direction in order to exhibit a sufficient shrinkage force in the width direction at heating.
Incidentally, it is desirable that a heat-shrinkable film have heat shrinkage properties such that the shrinkage gently increases with a rise in temperature. In the case where the shrinkage force rapidly increases on heating, since the shrinkage speed is too high, at the time of allowing a label with which a PET bottle or the like is covered to shrink in a steam tunnel, troubles such as distortion and jumping (allowing the label to shift upward) occur. On that account, an investigation of production conditions has been conducted so that the shrinkage gently increases with a rise in temperature.
For example, Non-Patent Document 1 describes that a film in which the shrinkage gently increases with a rise in temperature (the shrinkage speed is low) can be produced by allowing the drawing temperature in the width direction to be high and allowing the orientation in the shrinkage direction to be reduced or by allowing the heat treatment temperature after drawing in the width direction to be high, allowing the orientation in the main shrinkage direction to be fixed and allowing the shrinkage at the low temperature side to be reduced.
However, although it is possible to allow the shrinkage speed to be low by the production method described in Non-Patent Document 1, the shrinkage stress is reduced at the same time, and therefore in the case where the film is used as a label for a PET bottle or the like, the slack of the label after shrinkage and the like are generated and this is not preferred even from the viewpoints of the performance and the appearance.
On the other hand, the present applicant has consistently continued the development of a heat-shrinkable polyester film and has published a number of techniques. For example, Patent Document 1 discloses a heat-shrinkable polyester film satisfactory in perforation openability. However, there is no respite in user's demands, and a heat-shrinkable polyester film further excellent in shrinkage finishing properties has been desired.